The invention relates to a process for the fractionation of a gaseous mixture containing hydrogen, light aliphatic hydrocarbons and light aromatic hydrocarbons.
The invention more particularly aims at separately collecting (1) high purity hydrogen and in particular only containing traces of aromatic hydrocarbons, (2) C.sub.2 -C.sub.5 and in particular C.sub.3 or C.sub.3 -C.sub.4 aliphatic hydrocarbons, which can at least partly be recycled to a hydrocarbon conversion process, e.g. a dehydrocyclodimerization process, and (3) light aromatic hydrocarbons alone or in mixed form (BTX).
Various processes are known in which the effluent is a mixture of hydrogen, light and in particular C.sub.1 -C.sub.5 aliphatic hydrocarbons and light aromatic hydrocarbons, particularly benzene, toluene and/or xylene or their mixtures (BTX). These processes include catalytic reforming, aromatization, dehydrogenation, dehydrocyclization, steam cracking and dehydrocyclodimerization. More particularly, in the latter process, light olefins or paraffins, e.g. C.sub.3 and C.sub.4 are converted into light aromatic hydrocarbons in contact with zeolitic catalysts.
The conversion of aliphatic hydrocarbons into aromatic hydrocarbons is e.g. described in U.S. Pat. Nos. 4133743, 4210519, 4233268 and 4172027.
The aromatization of aliphatic hydrocarbons into aromatic hydrocarbons is e.g. described in French patents 2634139 and 2634140.
As a result of these processes and in a conventional manner the hydrogen is separated in a high pressure separator and the hydrocarbons are separated in a series of distillation columns.
The use of perm-selective diaphragms for the separation of hydrogen from hydrocarbons has also been proposed, e.g. in U.S. Pat. Nos. 4180388, 4398926 and 4654047. The use of a perm-selective diaphragm and fractionation columns is described in U.S. Pat. No. 45488619. In the latter patent, the effluent of a dehydrocyclodimerization unit is firstly fractionated, the liquid fraction being distilled to collect the BTX and the gaseous fraction is compressed and then washed by aromatic hydrocarbons or C.sub.7 -C.sub.10 paraffinic hydrocarbons having an external origin.